FIG. 7(a) is a perspective view illustrating a prior art semiconductor laser device having a window structure, and FIGS. 7(b), 7(c) and 7(d) are cross-sectional views illustrating cross-sections of the semiconductor laser of FIG. 7(a) taken along lines VIIb--VIIb, VIIc--VIIc, VIId--VIId, respectively. In the figures, reference numeral 100 designates a semiconductor laser device having a length of 300 to 600 .mu.m in the laser resonator length direction and a width of about 300 .mu.m in the laser resonator width direction. Reference numeral 101 designates an n type GaAs semiconductor substrate, and reference numeral 102 designates an n type Al.sub.x2 Ga.sub.1-x2 As (Al composition ratio: x2 =0.5) lower cladding layer having a thickness of 1.5 to 2 .mu.m. Reference numeral 103 designates a quantum well layer structure including a triple-layer structure comprising three Al.sub.y2 Ga.sub.1-y2 As (Al composition ratio: y2=0.05) well layers (not shown) each about 80.ANG. thick and two Al.sub.z2 Ga.sub.1-z2 As (Al composition ratio: z2=0.3 to 0.35) barrier layers (not shown) each 50.ANG. to 80.ANG. thick, alternatingly laminated with each other, and two barrier layers each of 0.2 to 0.3 .mu.m thickness, placed at top and bottom of the quantum well structure layer 103, sandwiching the triple-layer structure. Reference numeral 104 designates a p type Al.sub.w2 Ga.sub.1-w2 As (Al composition ratio: w2=0.5) first upper cladding layer of 0.2 to 0.3 .mu.m thickness. Reference numeral 105 designates a p type Al.sub.q2 Ga.sub.1-q2 As (Al composition ratio: q2=0.7) etching stopper layer of about 200.ANG. thickness. Reference numeral 110 designates a p type Al.sub.t2 Ga.sub.1-t2 As (Al composition ratio: t2=0.5) second upper cladding layer of 0.8 to 1.3 .mu.m thickness. Reference numeral 111 designates a p type GaAs first contact layer of about 0.7 .mu.m thickness. Reference numeral 106 designates an n type GaAs current blocking layer of 1.0 to 2 .mu.m thickness. Reference numeral 107 designates a p type GaAs second contact layer of 2 to 3 .mu.m thickness. Reference numerals 108 and 109 designate a p side electrode and an n side electrode, respectively. Reference numeral 112 designates a ridge produced in a reverse-trapezoid configuration of about 4 .mu.m width in the laser resonator width direction closest to n side electrode 109 and of about 5 to 6 .mu.m width in the laser resonator width direction closest to p side electrode 108. Reference numeral 113 designates a region into which Zn is diffused. Reference numeral 114 designates a region in the quantum well layer structure 103 where the quantum well layer structure is disordered by Zn of about 50 .mu.m width in the resonator width direction.
FIGS. 8(a) to 8(d) are perspective views illustrating process steps in producing the prior art semiconductor laser having a window structure. In the figures, the same reference numerals as those in FIGS. 7(a) to 7(d) designate the same or corresponding elements. Reference numeral 121 designates a first insulating film, reference numeral 120 designates a second insulating film for producing the ridge, and reference numeral 125 designates Zn diffusion.
A description will be given of a method for producing this semiconductor laser. First, as shown in FIG. 8(a), a cladding layer 102, a quantum well layer structure 103, a first cladding layer 104, an etching stopper layer 105, a second cladding layer 110, and a first contact layer 111 are epitaxially grown successively in this order on the surface of a semiconductor substrate 101. Next, a first photoresist 121 is formed on the first contact layer 111. The first photoresist 121 is patterned to produce apertures in the vicinity of both the laser resonator facets, and Zn 125 is diffused into regions which are to be the resonator facets using the patterned photoresist 121 as a mask as shown in FIG. 8(b). The Zn diffusion concentration is 1.times.10.sup.19 to 1.times.10.sup.20 cm.sup.-3. Subsequently, the wafer is annealed to disorder the quantum well structure layer. The quantum well structure layer can be also disordered not by conducting annealing at this time but by heat generated at the crystal growth in a later process step after the Zn diffusion. Next, as shown in FIG. 8(c), after the first photoresist 121 is removed, the first contact layer 111 and the second cladding layer 110 are etched to have a stripe configuration up to the surface of the etching stopper layer 105 using the second photoresist 120 as a mask to form a ridge 112. Subsequently as shown in FIG. 8(d), a current blocking layer 106 is selectively grown on the etching stopper layer 105 at the both sides of the ridge 112 so as to bury the ridge 112, and the second photoresist 120 is removed. Thereafter, a second contact layer 107 and a p side electrode 108 are successively formed on the ridge 112 and the current blocking layer 106, and an n side electrode 109 is formed on the rear surface of the semiconductor substrate 101, thereby completing a semiconductor laser 100 shown in FIG. 7(a).
The semiconductor laser operates as follows. When a voltage is applied to the semiconductor laser device 100 making the p side electrode 108 plus and the n side electrode 109 minus, holes are injected into the quantum well layer structure 103 through the p type GaAs second contact layer 107, the p type GaAs first contact layer 111, the p type Al.sub.t2 Ga.sub.1-t2 As (t2=0.5) second cladding layer 110, and the p type Al.sub.w2 Ga.sub.1-w2 As (w2=0.5) first cladding layer 104, while electrons are injected into the quantum well layer structure 103 through the n type GaAs semiconductor substrate 101, and the n type Al.sub.x Ga.sub.1-x As (x=0.5) cladding layer 102. Then, recombination of the electrons and holes occurs in the quantum well layer structure 103, whereby induced light emission is generated. When the amount of the injected carriers is sufficiently large to generate light of a magnitude larger than the loss in the waveguide path, laser oscillation occurs.
A description will be given of the ridge structure. In the semiconductor laser 100 having the ridge structure shown in FIG. 7(a), in a region covered with the n type GaAs current blocking layer 106 other than the stripe-shaped ridge portion 112, p-n junctions are formed between the p type AlGaAs first cladding layer 104 and the n type GaAs current blocking layer 106 and between the p type GaAs second contact layer 107 and the n type GaAs current blocking layer 106. Therefore, even when a voltage is applied making the p side electrode 108 plus, no current flows in the region other than the ridge region 112 where the p-n-p junction is produced to form a reverse bias. In other words, the n type GaAs current blocking layer 106 literally functions to block the current flow. Accordingly, current flows only through the ridge region 112 and is concentrated to a region of the quantum well layer structure 103 close to the ridge reaching a large current density sufficient for laser oscillation. Furthermore, the n type GaAs current blocking layer 106 has a characteristic of absorbing laser light emitted in the quantum well layer structure 103, because the energy band gap of GaAs is smaller than the effective energy band gap of the quantum well layer structure 103 based on the quantization effect. Therefore, the laser light is subjected to strong absorption at both sides of the ridge region 112 and is concentrated only in the vicinity of the ridge region 112. As a result, laser light which has a stable single mode and a horizontal transverse mode, which is important among the semiconductor laser operation characteristics, is obtained.
Next, the window structure will be described. In an AlGaAs series semiconductor laser which emits laser light of a 0.8 .mu.m band wavelength, which is generally used as a light source for an optical disc device such as a compact disc, the maximum light output is limited to the light output at which the laser resonator facet destruction occurs. The laser resonator facet destruction is induced by the semiconductor laser active layer, which is melted by heat generated by laser light absorption at the facet region, that the laser resonator facet destruction is induced, so that no resonance occurs. Accordingly, in order to realize the high light output operation, a device preventing facet destruction even at high light output is required. To realize this, a structure that makes laser light difficult to be absorbed at the facet region, i.e., a window structure that is "transparent" to laser light in the laser resonator facet region is quite effective. This window structure is formed such that the energy band gap at the region in the vicinity of the laser resonator facet is higher than that at the active layer which emits laser light. In the semiconductor laser 100 shown in FIG. 7(a) in which the quantum well structure layer 103 serves as an active layer, this window structure is formed utilizing the disordering of the quantum well layer structure 103 by Zn diffusion as shown by the production method of FIG. 8(b).
FIG. 9(a) is a diagram illustrating a profile of aluminum composition ratio in the quantum well layer structure 103 before being disordered by Zn diffusion, and FIG. 9(b) is a diagram illustrating a profile of aluminum composition ratio in the quantum well layer structure 114 after being disordered by the Zn diffusion. In the figures, reference numerals 19a and 19b designate Al.sub.z Ga.sub.1-z As (0.3.ltoreq.z .ltoreq.0.35) barrier layers, reference numeral 18 designates an Al.sub.y Ga.sub.1-y As (y=0.05) well layer, reference numeral Al.sub.1 designates an aluminum composition ratio of the barrier layers 19a and 19b, reference numeral Al.sub.2 designates an aluminum composition ratio of the well layer 18, and reference numeral Al.sub.3 designates an aluminum composition ratio of the disordered region 114 in the quantum well layer structure 103.
When an impurity such as Zn or Si is diffused into the quantum well layer structure 103, atoms constituting the well layers 18 and the barrier layers 19a and 19b are mixed with each other. As a result, since the well layer 18 is thinner than the barrier layer 19b, the Al composition ratio Al.sub.3 of the quantum well layer structure 114 after the diffusion becomes almost equal to the Al composition ratio Al.sub.1 of the barrier layers 19a and 19b before the diffusion as shown in the FIGS. 9(a) and 9(b), and the effective energy band gap of the quantum well layer structure 103 becomes approximately equal to the energy band gap of the barrier layers 19a and 19b. Thus, the energy band gap of the quantum well structure 114 disordered by the Zn diffusion becomes larger than the effective energy band gap of the non-disordered quantum well layer structure 103, whereby a window structure "transparent" to laser light is obtained.
While the prior art ridge structure semiconductor laser having a window structure constituted as described above, is very effective in preventing the destruction of a laser resonator facet, it has the problems described below:
In the region in the vicinity of the laser resonator facet where Zn is diffused, the impurity concentration increases, resulting in a reduced resistance, whereby a current injected from the electrode flows made easily through this region, but because there is no quantum well structure layer emitting laser light in this Zn diffused region, the current flowing through this region becomes an unavailable current that makes no contribution to laser oscillation at all. As a result, in this prior art semiconductor laser having a window structure, this unavailable current, likely to flow, causes a quite high threshold current and a quite high operational current.